1. Field of the Invention
The present invention relates to digital baseband modulation/demodulation apparatuses. More particularly, the present invention relates to digital baseband modulation/demodulation apparatuses applicable to base stations or mobile stations in a CDMA (Code Division Multiple Access) cellular communication system in which spread spectrum modulation by QPSK is performed as a primary modulation method.
In the next generation cellular phone system represented by IMT-2000, it is required to realize large capacity packet transmission system called HSDPA (High Speed Downlink Packet Access). In addition, it is desired to miniaturize apparatuses used in the system and to reduce power consumption of the apparatuses.
2. Description of the Related Art
FIGS. 1A, 1B and FIG. 2 are diagrams for explaining a conventional technology. FIG. 1A is a block diagram showing a three level QPSK modulation apparatus applicable to the IMT-2000 standard. The number 11 indicates a spread modulation part for complex spreading a pair of an Ic component and a Qc component of an input digital signal by using spreading code Is and Qs, wherein Is corresponds to the I axis, and Qs corresponds to the Q axis. The number 12 shows a phase rotation part for rotating the phase of the output signal from the spread modulation part 11. FIG. 2 is a block diagram of a demodulation apparatus corresponding to the modulation apparatus shown in FIG. 1A. In FIG. 2, 45 indicates a phase reverse rotation part and 46 indicates a despread demodulation part.
In FIG. 1A, a complex spread output signal (I, Q) output from the spread modulation part 11 is represented as:I=Ic·Is−Qc·QsQ=Ic·Qs+Qc·Iswhere Ic is send information spread by channelization code Cd, Qc is source information spread by channelization code Cc, Is is spreading code for the I axis, and Qs is spreading code for the Q axis.
The pair of I and Q component signals can be-represented as complex representation as follows in which I corresponds to the real number axis and Q corresponds to the imaginary number axis:I+jQ=Ac·As·ej(φc+φs)where Ac is the amplitude of a signal Ic+jQc, φc is the phase of Ic+jQc, As is the amplitude of the signal Is+jQs, and φs is the phase of Is+jQs.
The spread modulation part 11 has a transmit interrupt function (DXT) formed by AND gates A1-A4. The phase rotation part 12 is a part of a transmit diversity function for transmitting the same source information signals by shifting the phases.
FIG. 1B shows constellations on a complex plain for the modulation apparatus. In the figure, (a) shows a case where there is no phase rotation, and the signal amplitude on I/Q axes takes three levels (1, 0, −1), and there are five signal points including the origin point (I, Q)=(0, 0). In the figure, (b) shows a case where there is a phase rotation of 45° in which there are 9 signal points.
However, if the constellation in which the phase is rotated is adopted, the amplitude (2) of a signal output only on the I axis or only on the Q axis by the spread modulation part is two times of the amplitude (1) of other signal points. That is, 2 bits are necessary in a signal amplitude part. This results in increasing of the circuit size and the power consumption of the baseband modulation/demodulation apparatus.